Radio access technologies for cellular mobile networks are continuously evolving to meet demands for higher data rates, improved coverage, and increased capacity. An example of recent evolution of Wideband Code-Division Multiple Access (WCDMA) technology is the so-called High-Speed Packet Access (HSPA) developed by the 3rd-Generation Partnership Project (3GPP). Further evolution of 3G systems is ongoing in 3GPP's Long Term Evolution (LTE) initiative, which includes the development and specification of new access technologies and new system architectures. An overview of the LTE system is provided in “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall Description, Stage 2, (Release 8)”, 3GPP TS 36.300, v. 8.2.0, September 2007, the contents of which are incorporated by reference herein.
One goal of the LTE initiative is that the access technology should be designed for flexibility, so that it may be used in existing frequency allocations as well as in new frequency allocations. This approach allows for easy introduction in spectrum with existing deployments. For similar reasons, LTE is designed for use with multiple duplexing solutions. Both FDD (Frequency Division Duplex) and TDD (Time Division Duplex), where uplink and downlink transmissions are separated in frequency and in time respectively, are supported, to permit usage of LTE technology with paired and unpaired spectrum allocations. Furthermore, to allow for even more flexibility in using available spectrum, LTE's access technology is based on OFDMA (Orthogonal Frequency Division Multiple Access) for the downlink and Single-Carrier Frequency Division Multiple Access (SC-FDMA) for the uplink. These technologies permit finely-grained, dynamic allocation of spectrum resources to uplink and downlink communications. Thus, available resources may be dynamically adjusted based on individual user requirements as well as aggregate demand.
In wireless communication systems in general, transmitting at excessive power levels (e.g., at power levels greater than necessary to maintain a desired quality of service) should be avoided. This is generally desirable to avoid interference with other transmitted signals, and is especially desirable in a mobile terminal to maximize the time between recharges for the terminal's battery. The LTE specifications thus support a power control mechanism wherein a serving base station (an evolved Node-B, or eNodeB, in 3GPP terminology) controls a mobile terminal's transmitter output power.
The basic contours of a power control mechanism for LTE are provided in “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures,” 3GPP TS 36.213, v. 8.1.0, dated Dec. 12, 2007, the contents of which are incorporated herein by reference. The defined mechanism provides that the power setting for each mobile terminal sub-frame transmission is calculated as a function of the bandwidth allocated for the sub-frame, the modulation and coding scheme allocated for the sub-frame, and a current path loss estimate. In some operating modes, the transmitter output power is further calculated as a function of a parameter representing accumulated transmit power control (TPC) commands received by the mobile terminal.
This preliminary power control mechanism specified by 3GPP is designed to address the dynamic scheduling allowed in an LTE system. The bandwidth and modulation scheme employed by the mobile terminal may change from one sub-frame to the next—to avoid transmitting at excessive power levels, the transmitter output power level must vary with these changes in resource allocations. The transmitter output power level is also dynamically adjusted to accommodate changes in propagation changes, e.g., transmission path loss. However, the power control mechanism described in the above-mentioned 3GPP specification does not handle power-limited situations adequately.
Problems with transmit power control mechanisms in power-limited situations have been recognized in other wireless communication systems. For instance, U.S. Patent Publication No. 2006/0050798, by Odigie et al., dated Mar. 9, 2006, describes the operation of a transmit power control system in power-limited circumstances for a Wideband Code-Division Multiple Access (W-CDMA) system. However, the methods and apparatus disclosed by Odigie do not address the dynamic resource scheduling permitted in LTE systems. Furthermore, the systems disclosed in Odigie do not use an accumulated TPC command parameter as required by the LTE specifications.